make RIFoo.providedBy(rref) work

[Brian Warner]

it would be nice to be able to do:

 def set_remote(self, rref):
     assert RIFoo.providedBy(rref)
     self._remote = rref

and have that assert two things: 'rref' is a RemoteReference?, and that RemoteReference? is associated with a Referenceable that claims to implement RIFoo. If set_remote were remote_set_remote, we could do this with a schema check, but we don't currently have the ability to test this sort of thing outside (un)serialization time.